The project's long-term objective is to understand how progression through the cell cycle is regulated in eukaryotic cells. As a model system, we are studying the control regulating the initiation of mitosis in the fission yeast, Schizosaccharomyces pombe. This project focuses on one element of the mitotic control, the weel+ protein kinase, that functions as a dose-dependent inhibitor of mitosis. We have three major aims. First, by using weel+ immunoprecipitates to assay protein kinase activity, and by constructing mutations at sites essential for protein kinase activity, we will determine if weel+ function is mediated by protein phosphorylation. This will establish if protein kinases can function as negative cell cycle control elements. Our second aim is to determine how activity of the weel+ mitotic inhibitor is regulated during the cell cycle and the role that this plays in the mitotic control. We will concentrate first on the possibilities that weel+ protein is produced or degraded periodically during the cell cycle, or that weel+ protein activity is regulated by phosphorylation in a cell cyclic fashion. Our third aim is to identify elements that are closely involved with the function of the weel+ mitotic inhibitor. These will be identified in a screen for pseudorevertants, in which we select for mutations that allow cells to overcome a cell division arrest caused by overexpression of weel. These studies will significantly advance our understanding of cell cycle control in a simple eukaryote, and provide an important framework for analysis of the more complex controls operating in multicellular organisms.